1. Field of the Invention
This invention relates to a test apparatus and a method of making precise thermal performance measurements of thermal insulation specimens, and more specifically to a comparative testing method and apparatus for evaluating thermal insulation materials and composite systems in flat plate configurations for low temperature applications.
2. Relevant Art
U.S. Pat. No. 3,242,716 is directed to a method and apparatus for testing thermal conductivity of insulation placed over a test apparatus. This device employs separate passageways for introducing a cryogenic fluid into the test chamber and disposing of the boiled-off gases. The method of testing is consistent with a boil off calorimeter, i.e., the absolute method as set forth in ASTM procedure C745. To perform the testing procedure according to the invention, an extended amount of time of carefully controlled monitoring may be necessary to complete the test. In comparison, the present invention follows the xe2x80x9ccomparative methodxe2x80x9d including controlled parasitic heat leak in which no monitoring of the testing process is required. Once the testing process is initiated, it will continue without the need for intervention until the cryogenic liquid is completely boiled-off, allowing for precise measurement of the thermal properties of the insulation material.
U.S. Pat. No. 4,396,300 is directed to a test apparatus for testing the heat transfer and friction characteristics of a tube. It does not teach how to test a flat thermal insulation specimen.
Precise measurement of thermal performance of a thermal insulation system is advantageous in many applications. As technology advances, the use of cryogenics has become more and more commonplace. As an example, hydrogen may become the common fuel source. An infrastructure for the storage, distribution and handling of liquid hydrogen would then be needed for many applications. This demand for the storage and handling of various cryogens will require the development of highly effective thermal insulation systems.
At least three ASTM procedures exist for testing thermal properties flat slab specimens: C177 (absolute method, guarded hot plate), C518 (comparative method, heat flow meter), and C745 (absolute method, boil off calorimeter). C177 is difficult to apply for a large temperature differential (high delta T) and in vacuum conditions. The document summary provided by ASTM for this procedure specifically warns that special precautions are required for conducting the test under vacuum conditions. C518 is used mostly for near ambient temperature testing of materials such as building insulation and requires the measurement of specimens of well known thermal transmission properties to calibrate the apparatus. C745 is very difficult in set-up and execution in part because of the necessity of providing a cryogenic guard vessel. These methods are typically limited to small temperature differences (such as 20K) and a limited range of vacuum pressure.
One valuable technique for testing the thermal performance of materials, preferably insulation material, is boil-off testing. Boil-off testing is accomplished by filling a vessel with a fluid that boils, or evaporates, below ambient temperature. Cryogens such as liquid nitrogen, liquid helium, liquid methane, liquid hydrogen, or other known refrigerants may be used. The vessel is placed on top of the testing material. The vessel (cold mass tank) is then filled with the cryogenic liquid. A calorimetry method is then used to determine the thermal conductivity of the testing material by determining the amount of heat that passes through the test material to the vessel containing cryogenic liquid. The cryogenic liquid boil-off rate from the vessel is directly proportional to the heat leak rate passing through the test material to the cryogenic liquid in the vessel. For a test material under a set vacuum pressure, the apparent thermal conductivity (k-value) is determined by measuring the flow rate of cryogenic boil-off gas at given warm and cold boundary temperatures across the thickness of the sample.
Although known cryogenic boil-off techniques and devices have been utilized to determine the thermal conductivity of insulation material, the previous techniques and devices have proven less than successful for a variety of reasons. First, few such cryogenic devices are in operation because of their impracticality from an engineering point of view. Previous cryogenic boil-off devices made it extremely difficult to obtain accurate, stable thermal measurements and required extremely long set up times. Prior testing devices also needed highly skilled personnel that could oversee the operation of the cryogen testing device for extended periods of time, often exceeding 24 hours to many days in some cases. Additionally, constant attention was required to operate previous cryogenic testing devices to make the necessary fine adjustments required of the testing apparatus. Second, the testing of high performance materials such a multilayer insulation requires extreme care during installation. Inconsistency in performing installation techniques is a dominant source of error and poses a basic problem in the comparison of such materials. Localized compression effects, sensor installation, and out-gassing provided further complications. Third, measurements of various testing parameters were not carefully determined or controlled in known testing devices. Measurement of temperature profiles for insulation material was either not done or was minimal because of the practical difficulties associated with the placement, feed-though, and calibration of the temperature sensors. Vacuum levels were restricted to one or two set points or not actively controlled altogether. Fourth, previous cryogenic testing devices required complex thermal guards having cryogenic fluid filled chambers to reduce unwanted heat leaks (end effects) to a tolerable level. The previous technique for providing thermal guards, i.e., filling guard chambers with the cryogen, caused much complexity both in construction and operation of the apparatus. Known techniques added the further complication of heat transfer between the test chamber and the guard chambers due to thermal stratification of the liquid within the chambers.
Accordingly, there exists a need for a method, preferably as simple as possible, for evaluating the thermal performance of materials under actual use conditions. These conditions include large temperature differences (as much as 200K) and a full range of vacuum pressure levels (between approximately 1xc3x9710xe2x88x925 torr to approximately 1000 torr).
Consequently, it is a primary object of the present invention to provide a method and apparatus for obtaining data to measure the heat leak rate through materials under actual use conditions such as temperature, vacuum, environment, and/or mechanical loading.
It is a further object of the present invention to provide a method and apparatus for providing comparative heat leak rates and comparative apparent thermal conductivity values.
Another object of the present invention is to provide a method and apparatus which is easily employed to provide the thermal performance of actual specimens.
Accordingly, the present invention provides a test apparatus and method to measure the heat leak rate through materials.
To eliminate or minimize the foregoing and other problems, a new system and method of testing thermal insulation systems has been developed. In particular, the present invention overcomes the foregoing problems by providing a cryogenic testing apparatus having a boil-off calorimeter system for calibrated measurement of the apparent thermal conductivity (k-value) of a testing material, preferably insulation material, at a fixed vacuum level. Vacuum levels can range from high vacuum (below 1xc3x9710xe2x88x925 torr) to soft vacuum (about 1 to 10 torr) to no vacuum (above 760 torr).
The test apparatus is preferably comprised of a test assembly having a single liquid nitrogen feed through assembly to fill liquid nitrogen into a cavity formed in a cold mass tank and subsequently vent nitrogen gas as it boils-off of the liquid nitrogen present in the cold mass tank cavity. The cold mass tank cavity receives cryogenic liquid from the feed through assembly wherein the cryogenic liquid contacts a side of the cold mass tank having contact surface directly contacting a test specimen mounted in the test apparatus. The cold mass tank and the test specimen are preferably contained within a vacuum chamber. Compressions rods may be utilized to press the cold mass tank into contact with the test specimen, providing a measurable mechanical loading to the test specimen, thereby controlling the thickness of the test specimen. Insulation and shield guards may surround the test specimen to reduce the at transfer from the sides of the test specimen to minimal and repeat able levels.
A heater may be used to apply a desired temperature on a first or heated side of the test specimen while the cold mass eventually achieves and maintains a steady state temperature, for example about 80K, on the cold or second side of the test specimen. The heat transfer through the test specimen boils off, or evaporates, the liquid nitrogen in the cold mass tank that can be measured by a mass flow meter. When the boil-off is complete and stability is reached, the thermal conductivity of the test specimen may be calculated from temperature and mass flow rate data. The vacuum environment may be established along with the mechanical loading and charts showing the thermal performance of the test specimen under loading and various vacuum conditions may be created for future use.